The present invention relates to an improvement in the process for forming a thin film of an inorganic substance on the surface of a substrate by the method of sputtering. More particularly, the invention relates to an improvement in the process of film-forming of an inorganic substance on the surface of a substrate, as in the method for the preparation of a membrane to serve as a mask for X-ray lithography, by the sputtering method to cause deposition of the inorganic substance on the substrate surface.
As is known, the method of X-ray lithography is highlighted as a promising technoilogy in the near future for fine patterning in the manufacture of various kinds of semiconductor devices along with the recent trend toward more and more increased fineness in patterning. The X-ray lithography is performed by using a membrane of an X-ray permeable inorganic substance such as boron nitride, silicon carbide, silicon nitride and the like as a mask bearing a pattern formed of an X-ray-impermeable substance. Such a membrane of an X-ray-permeable substance is prepared by the deposition of a thin film of the substance on the surface of a substrate by the method of chemical vapor-phase deposition (CVD) or sputtering followed by removal of the substrate to leave the thin film in the form of an unsupported but framed membrane.
Each of the above mentioned CVD method and sputtering method has its own advantages and disadvantages as a method for film-forming on the substrate surface. For example, the CVD method, in which the film is deposited on the substrate surface by the reaction and decomposition of the gaseous reactant materials in the vapor phase, is subject to a problem that impurities, such as an undesirable element and the like, formed in the vapor-phase reaction are sometimes taken into the deposited film so as not to give a satisfactory membrane due to occurrence of pinholes and nodules on the surface while such an impurity in the membrane of the X-ray mask is sometimes removed by the irradiation with X-rays so that the membrane is subject to various defects such as appearance of distortion, variation in the stress, decrease in the mechanical strengths, decrease in the optical transparency and so on.
The sputtering method, on the other hand, has another problem. In the high-frequency magnetron sputtering method which is undertaken in most cases of sputtering in view of the high efficiency as compared with other types of sputtering method, the thin film deposited on the substrate surface is not very uniform on the whole area in respect of the internal stress. It is usual as a trend that, when sputtering is performed with parallel-disc electrodes, the internal stress is lower in the central region of the deposited film, which faces the center of the target during sputtering, than in the peripheral region. When a membrane is prepared from such an unevenly stressed film deposited on the substrate surface, rupture of the membrane sometimes starts at the central region under a lower stress or creases or wrinkles are sometimes formed in the central region greatly affecting the quality of the X-ray lithographic mask. Accordingly, it is eagerly desired to develop an improved sputtering method by which high uniformity is ensured in the internal stress of the sputtering-deposited film on a substrate surface from which high-quality X-ray lithographic masks can be prepared in a high yield.